PROJECT SUMMARY Ribosomes carry out protein synthesis in all cells, interpreting the information contained in the mRNA to produce the proper amount of the correct protein. In addition, ribosomes also mediate mRNA quality control. Thus, misassembled ribosomes can affect the sequence and abundance of proteins and mRNAs, thereby disrupting protein homeostasis. This can lead to a number of diseases, demonstrating the importance of ensuring ribosomes are accurately assembled, and produced in the correct numbers. Using a combination of biochemical, genetic, genomic and structural tools, we will (i) investigate mechanisms that promote proper incorporation of ribosomal proteins and folding of the RNA, (ii) dissect quality control pathways to identify and ultimately degrade misassembled intermediates and (iii) study how misassembled ribosomes promote disease. In the first part, we will build on our existing work and study the late assembly of the ribosomal head, as well as combine insights from recent structures and our biochemical work to understand how DEAD-box proteins are used to construct ATP-dependent regulatory switches to control major conformational transitions in early 40S biogenesis. In the second part, we will extend our work on quality control to investigate a possible proofreading mechanism and identify degradation pathways for misassembled intermediates, a novel frontier for the field. In the last part, we will investigate how ribosomes containing substoichiometric levels of two ribosomal proteins, Asc1 and Rps10, which are produced in cancer cells that lack sufficient amounts of the assembly factor Ltv1, promote disease. This work builds on a genetic system we have developed to separate ribosomes of distinct composition, and also takes into consideration the known roles of these proteins in mRNA quality control. In addition, we will also investigate the effects from dysregulation of ribosome numbers in disease. Together, the proposed work will link mechanistic insights into a fundamental problem of cell and molecular biology ? how ribosomes are assembled, to human disease.